In methods and arrangements for frequency conversion of known art high intensity, coherent electromagnetic radiation with an adjustable wavelength in the range between about 680 nm and 1020 nm is coupled into the converter arrangement. Based on the principle of non-linear optical three-wave mixing, in particular of non-linear optical generation of higher harmonics of the output frequency, the fundamental radiation is transformed in the converter arrangement into radiation in the wavelength range between 205 nm and 510 nm. For this purpose three different conversion processes are used depending on the wavelength to be generated, and in particular for the generation of electromagnetic radiation in the wavelength ranges:                from about 340 nm up to about 510 nm, a frequency doubling (SHG: second harmonic generation),        from about 250 nm up to about 340 nm, a frequency tripling (THG: third harmonic generation),        from about 205 nm up to about 250 nm, a frequency quadrupling (FHG: fourth harmonic generation),        
This fundamental principle is also used in the method and arrangement of the present invention. The boundaries between the individual ranges, as well as the upper boundary of the frequency doubling range, can be displaced by about 10 nm commensurate with the properties of the radiation source used on the input side. The lower boundary of the frequency quadrupling range is based on physical properties of the non-linear optical material BBO (beta barium borate) that as a rule is currently used for a frequency quadrupling. Shorter wavelengths down to about 190 nm can be generated on the output side in an additional converter stage by way of non-linear optical sum frequency generation of the frequency tripled radiation with non-converted components of the fundamental radiation.
Frequency tripling takes place by way of a two-stage non-linear optical process. Firstly, a frequency doubling of a proportion of the fundamental radiation is carried out in a first non-linear optical crystal, the doubler crystal. Then the sum frequency of the frequency doubled radiation generated in the first crystal and the residual fundamental radiation at the output frequency is generated in a second non-linear optical crystal, the tripler crystal. Up to the present time two different conversion schemes have been used for this purpose:    1. SHG type I (ooe) and THG type I (ooe):            (750-1020) nm|o>+(750-1020) nm|o>→(375-510) nm|e>        (750-1020) nm|o>+(375-510) nm|o>→(250-340) nm|e>            2. SHG type I (ooe) and THG type II (eoe):            (750-1020) nm|o>+(750-1020) nm|o>→(375-510) nm|e>        (750-1020) nm|e>+(375-510) nm|o>→(250-340) nm|e>where the ordinary polarised radiation component in the crystal in question is designated by “o” and the extraordinary polarised radiation component is designated by “e”. Suitable optics must be introduced for the necessary polarisation rotation between the two stages of these processes. However in today's prior art no optics of an adequate quality are available for purposes of simultaneous beam shaping or independent modification of the polarisation of the two frequency bands.        
There therefore remains on the one hand the option of separating the two radiation fields into the different frequency bands downstream of the first crystal, then to deal with them separately, i.e. to introduce optics to influence beam shape and polarisation, and subsequently to combine them once again upstream of the second crystal. An example of a procedure of this kind can be found in U.S. Pat. No. 6,816,520 B1.
The second, more advantageous option consists in selecting the orientation of the two crystals such that no additional optics are required between the two crystals. This is described, for example, in G A Rines et al., “Non-linear Conversion of Ti:Sapphire Laser Wavelengths”, IEEE Journal of Selected Topics in Quantum Electronics, Vol. 1, No. 1, April 1995, Pages 50 to 57. Here the principal section of the tripler crystal is rotated in the transverse direction through 90° relative to the principal section of the doubler crystal, so as to maintain the appropriate conversion condition with the correct polarisation direction of the radiation fields relative to the orientation of the crystal axis. However, this is only possible with the second of the conversion schemes cited above. The first option, in contrast, can be introduced in principle using both conversion schemes, but has the disadvantage of high sensitivity to alterations of direction of the input beam. Because of dispersive refraction, alterations of direction of this kind lead to a relative displacement of the two beams in the tripler crystal and thus to a lower conversion efficiency.
The second option is advantageous since a near-perfect spatial superposition of fundamental and frequency-doubled radiation inherently exists downstream of the doubler crystal, which is preserved up to entry into the tripler crystal. However, the rotation of the tripler crystal relative to the doubler crystal leads to a restriction in the conversion of broadband tuneable radiation. For radiation that can be tuned in wavelength the crystals must be additionally rotated in the plane of the principal section, in order to fulfil the phase matching necessary for an efficient conversion process for the different wavelengths by way of so-called angle tuning. With an orientation of doubler crystal and tripler crystal rotated by 90° these crystals must then be rotated in two planes at right-angles to one another when adjusting for different wavelengths of the fundamental radiation. This leads to an arrangement that is not very compact.
Furthermore on account of the parallel displacement of the beams generated during rotation of the crystals an arrangement of this kind requires tripler crystals with large apertures. These are correspondingly expensive. The larger volume in combination with the relatively poor thermal conductivity also exacerbates the maintenance of an even temperature in the crystals. As a result of the crossed orientation of the non-linear crystals a migration of the extraordinary polarised radiation fields occurs in both transverse directions, called walk-off. In order to reduce the consequences of this effect in terms of limiting efficiency and reducing beam quality, the beam cross-section in the crystals must have a comparatively large semi-axis in both transverse directions. For a given input power, however, this limits the intensity of the radiation field that determines efficiency. The second of the conversion schemes cited above is therefore deployed primarily for laser sources with a large pulse power and a low repetition rate (less than 100 Hz), but not for more modern lasers with a high repetition rate (more than 1 kHz) and a comparatively small pulse power.
M Ghotbi et al., “Efficient generation of high-energy picosecond pulses at 355 nm in BiB3O6”, 2005 Conference on Lasers and Electro-Optics Europe, Page 238, disclose a method for multi-stage frequency conversion in which a positively birefringent crystal with a type I phase matching is introduced for both first and second crystals. Frequency conversion takes place in both crystals according to the (eeo) conversion scheme. Here the two crystals must be rotated in different spatial planes for purposes of a phase matching.
The object of the current invention consists in specifying a method and also an arrangement for the frequency conversion of coherent optical radiation, that can be tuned in wavelength, in a two-stage non-linear optical process, which enable a high conversion efficiency with a compact mode of construction of the arrangement.